Spread spectrum technology has been known for some time. A spread spectrum system is one in which the transmitted signal is spread over a wide frequency band. Typically, the frequency band over which the transmitted signal is spread is wider, by a substantial margin, than the minimum bandwidth required to transmit the information being sent. For example, a spread spectrum system may convert a base-band signal having a bandwidth of a few kilo-Hertz (kHz) to a spread spectrum signal having a bandwidth of many Giga-Hertz (GHz), and transmit the signal using that wider bandwidth.
Spread spectrum signals are typically generated using code modulation such as frequency hopping or direct sequence coding. Because of this code modulation, the power transmitted is low in any narrow frequency region. The power density of a spread spectrum signal is far less than that of more conventional signals in which all the transmitted power is sent in a band of frequencies commensurate with the baseband information bandwidth.
Spread spectrum communication provides a number of advantages over more conventional types of communication systems. These advantages include selective addressing capability, coupled access for code division multiplexing, message screening from eavesdroppers and low-density power spectra for signal hiding. These characteristics are useful and highly desirable in many types of radio communication systems. These and other characteristics of spread spectrum technology are described in SPREAD SPECTRUM SYSTEMS, edited by Robert C. Dixon, published by John Wiley & Sons, Inc. (1984).
For purposes of background information dealing with conventional spread spectrum systems, as well as for supplemental information dealing with modular yet conventional segments of the illustrative embodiments set forth herein, the reader's attention is directed to the following references: U.S. Pat. Nos. 4,455,651, and 4,086,504, dealing with wireless systems employing spread spectrum techniques; U.S. Pat. No. 4,438,519, dealing with a narrow bandwidth spread spectrum system using an AC power line as an antenna; U.S. Pat. No. 4,475,208, concerning a voice/data spread spectrum system; U.S. Pat. No. 4,320,514, dealing with a spread spectrum radio receiver for frequency hopped signals; and U.S. Pat. No. 4,672,605, dealing with a data/voice communication system based on spectrum spreading.
One rather unique type of radio communication system employs a radiating (or "leaky") transmission line, such as leaky coaxial cable, throughout a confined area to provide coverage in that area. Leaky transmission line is advantageous, because it provides for total coverage of a restricted area. Thus, it is not uncommon to find leaky transmission line in tunnels, offices and other areas that are frequency-blocked from externally located wide area coverage communication systems.
Combining leaky transmission line with spread spectrum technology in a single system, to realize the advantages of both technologies, has been recognized as a worthwhile objective. For example, in U.S. Pat. No. 4,866,732 these technologies are described as part of a single system having radio units communicating over a radiating transmission line, with each radio unit communicating using a uniquely assigned spreading code. Unfortunately, there are problems associated with the implementation of this technological combination which have not been overcome.
A fundamental problem with such a system involves regulations defined by the Federal Communications Commission (FCC). The FCC has allocated certain bands for use of spread spectrum radio transmissions having a power level of up to one Watt. These FCC bands are commonly referred to as the Industrial, Scientific and Medical (ISM) bands and are respectively assigned to the frequency ranges of 902-928 MHz, 2400-2483.5 MHz and 5725-5850 MHz. Although operation at 2400-2483.5 MHz is conceivable with leaky coaxial cable, there remains a number of unsolved problems concerning coupling losses and attenuation in leaky coaxial cable at this frequency. Moreover, due to attenuation, leaky coaxial cable is not typically useful for frequency transmissions in excess of 2 GHz. Thus, the most practical ISM band is in the 902-928 MHz region.
The 26 MHz bandwidth in the 902-928 MHz region is, however, narrow and complicates full-duplex spread spectrum communication over leaky coaxial cable. This is because a 26 MHz bandwidth does not allow for the provision of sufficient isolation between the transmit and receive signals without the aid of extensive filtering tools, which can be expensive and occupy an excessive amount of space.
Accordingly, there is a need for a spread spectrum communication system using leaky coaxial cable that can be implemented in a cost effective manner while overcoming the afore-mentioned problems.